1. Field of the Invention
The present invention relates to a heat exchanger having a porous metal, in which thermal energy of exhaust gases is available for the thermal decomposition of natural gas to produce reformed fuel, the generation of steam from water, the condensing of a vapor to a liquid, the warming of an oily substance, and so on.
2. Description of the Prior Art
Among conventional systems for reclaiming heat energy from exhaust gases emitted out of any heat resource including engines, combustion chambers and so on are commonly any systems where an exhaust gas turbine is used to convert the exhaust heat to either kinetic energy or electric energy. With the prior turbo-compound systems in which the exhaust turbine is connected to the exhaust pipe from the engine, nevertheless, any excessive increase in ingress pressure of the exhaust gas turbine would cause heavy loads on the exhaust phase of the engine, resulting in adversely giving rise to any loss in power. To cope with this, it has been preferred that the exhaust gas turbine is less in ingress pressure while the energy conversion is made using steam power.
Some sort of a heat exchanger disclosed in, for example Japanese Patent Laid-Open No. 6601/1999 is known, in which a porous ceramics member is installed in a gas passage while first-stage and second-stage heat exchangers are provided in the course of an exhaust line out of a gas engine to boost the steam in temperature. The first-stage heat exchanger is constituted with a steam passage installed in a first casing to allow the steam to flow through there, and an exhaust gas passage arranged in the steam passage to get the exhaust gases running through there. The second-stage heat exchanger includes a water-steam line allowed to hold water therein, which is installed in a second casing lying behind the first casing, and an exhaust gas line surrounding around the water-steam line to allow the exhaust gases to flow through there.
A natural gas-reforming system disclosed in, for example Japanese Patent Laid-Open No. 93777/1999 is also known, in which the principal constituent: CH4 in natural gas is pyrolyzed to the reformed fuel of CO and H2 to improve the gas engine in thermal efficiency, and further the CO2 contained in the exhaust gases is used for the pyrolysis, thus rendering the CO2 content in the exhaust gases reduced. With the fuel-reforming system recited earlier, an exhaust gas passage is defined inside an exhaust gas tube while a gaseous fuel casing is disposed around the exhaust gas tube to allow the gaseous fuel to flow through there. The gaseous fuel casing is filled with porous ceramic substance coated with a catalyst helping convert the CH4 in natural gas into CO and H2. In addition, the gaseous fuel casing is shielded around there with a thermal insulation.
Another sort of a gas engine with fuel-reforming system disclosed in, for example Japanese Patent Laid-Open No. 13547/1999 is known, in which the principal constituent of CH4 in natural gas is pyrolyzed to CO and H2 to improve the gaseous fuel in calorific value, while the CO2 contained in the exhaust gases is used for the pyrolysis, thereby the CO2 content in the exhaust gases getting reduced and further the formation of NOx is curbed. With the gas engine constructed as stated earlier, a mixture of CH4 with CO2 is fed into a catalytic converter installed in the exhaust pipe, where the gaseous mixture is heated and pyrolyzed with the exhaust gases to convert into reformed fuel. The CO2 separated out from the exhaust gases though a separator membrane is forced into the catalytic converter. Heat energy remaining in the exhaust gases is reclaimed at a turbo-charger and also discharged at the first and second heat exchangers to produce high-temperature steam that is in turn used to drive a steam turbine, which would result in reclaiming the heat energy as electric energy.
A steam engine working on Rankine cycle disclosed in, for example Japanese Patent Laid-Open No. 51582/1999 is also known which is comprised of a steam generator to convert the liquid to vapor, a steam turbine driven with the vapor produced in the steam generator, a condenser to reduce exhaust steam from the steam turbine to a liquid, and a pump to return the liquid discharged out of the condenser back to the steam generator. The condenser is composed of an inside cylinder providing a fluid passage to allow the steam leaving the steam turbine to flow through there, the inside cylinder having a rotor of permanent magnet, a first porous member installed in the fluid passage, a second porous member wound around the inside cylinder in a spiral way to form successive fins, and an outside cylinder surrounding around the successive fins to provide an air passage in which any one fin and a circular space separating any two successive fins alternate lengthwise within the outer cylinder, the outer cylinder having a stator in opposition to the rotor on the inside cylinder to bear the inside cylinder for rotation thereon.
A gas engine disclosed in, for example Japanese Patent Laid-Open No. 6602/1999 is also known, in which an energy recovery means with heat exchanger is disposed behind a turbocharger installed in an exhaust pipe. High-temperature steam produced in the heat exchanger passes through a steam turbine to produce electric power by the action of a generator coupled with the steam turbine. The gas engine employs fuel of natural gas and is applicable well to, for example a cogeneration system. The gas engine includes a fuel tank to hold a natural gas containing a principal constituent of CH4, a fuel pump to forcibly feed the gaseous fuel into an auxiliary chamber connected to a main combustion chamber, a first heat exchanger unit installed behind the turbocharger in the exhaust pipe, a steam turbine driven by the steam produced in the first heat exchanger unit, and a second heat exchanger unit disposed behind the first heat exchanger unit to convert a low-temperature vapor and water leaving the steam turbine into a high-temperature vapor that is fed back to the first heat exchanger unit. The generator, when driven by-the steam turbine, produces electric power in proportion to turning force exerted by the turbine.
Meanwhile, a heat exchanger made of ceramics of porous texture has been developed in late years. Nevertheless, the ceramic products, because of vulnerable to impact stress and therefore less in fracture toughness, are very unfit to employ them for the heat exchanger that is commonly sophisticated in configuration. To cope with this, the advent of a heat exchanger of porous metal has been expected until now. Production of the heat exchanger with porous metal, however, has been tried with little success so far because porous metal is very tough to join it with solid metal sheet.
Moreover, the heat exchanger should be high in efficiency for the reclaiming of heat energy from the exhaust gases. With the engines of turbo-compound type employing the fuel of natural gas, the combustion chamber has to be made in heat insulation to exploit the most of heat energy from the exhaust gases, converting the most of energy derived from the fuel into power. Effectiveness in the heat exchanger is very crucial for the heat transfer from one fluid to another. That is, the higher the effectiveness in the heat exchanger is, the better it is for available rate of heat energy and therefore for the overall thermal efficiency. The operating fluids have considerable affect on the effectiveness of the heat exchanger in both their heat conductivity and heat transfer rate, and also less thermal resistance is preferred for smooth mobility of heat.
In recent years, many advanced researches in foamed heat-resisting metals higher in heat conductivity have focused attention on the production of porous metallic members that are preferable for diverse uses including filters and so on. The porous metallic product has a complex geometrical construction in which metals get entangled and intersected with one another in three-dimensional structure, and therefore has the outside surface area per unit volume, which is up to about six times greater than the conventional fins and further made continuous over the product block. This feature is fit well for heat transfer between the fluids that are different in temperature from one another.
Thus, the present inventor has led to the concept that porous metallic members are joined together with opposite sides of metallic sheet, one to each side, which is a partition wall to separate two fluids at different temperatures from one another to provide a heat-extracting area or hotter area and a heat-emitting area or colder area in opposition to each other across the partition wall. When the hot fluid including a hot gas and so on passes over the heat-extracting area or hotter area through clearances in the associated porous metallic member with coming into collision contact against the over-all surface of the porous metallic member, the remaining heat in the hot fluid is first transferred to the solid of the porous metallic member, and then to the wall of metallic sheet. The heat is eventually transmitted to another fluid in the heat-emitting area or colder area. To make certain of smooth heat transmission between the two fluids, it would be conceived that the porous metallic members have to be securely joined together with the wall through their stems that come in engagement with the sides of the wall.
Moreover, it is necessary to employ the heat exchanger high in efficiency in order to realize the effective reclaiming of heat energy from the exhaust gases. With the engines of turbo-compound type employing the fuel of natural gas, the combustion chamber needs heat insulation to exploit the most of heat energy from the exhaust gases, converting the most of energy derived from the fuel into power. Effectiveness in the heat exchanger is very crucial for the heat transfer from one fluid to another. That is, the higher the effectiveness in the heat exchanger is, the better it is for available rate of heat energy and therefore for the over-all thermal efficiency. The operating fluids have considerable affect on the effectiveness of the heat exchanger in their heat conductivity and heat transfer rate, and also less thermal resistance is preferred for smooth transmission of heat.